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Old 31st May 2010, 12:43   #1
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Default BHP and top speed aka "Transmission efficiency"

It has always intrigued me that we still don't assess enough of cars. Of course there can be no end to it. But since it's being done, have to take it further. smartcat's nice statistical study and ranking made for interesting reading - and I must congratulate him on the amazing effort.

To what is on top of my mind, let me explain. The Maruti Swift D and the Getz CRDi were compared and the Swift was declared the winner long back. This kind of an opinion eventually killed the Getz CRDi although it was a kind of a collector's item. The point is. The Swift had 75bhp and the Getz had 110 (or 108 as some reviwers said , you never knew the exact truth!) horses at its command. The Getz had a top speed of 171 kph, while the Swift with much less power could do 160 comfortably (actual speeds, not speedo readings).
To some extent the final drive ratios in the Getz were to blame, there was hearsay that Hyundai never changed the gear ratios for the GetzD believing probably that the engine would do the rest or whatever, so the full power could never be harnessed.Or they did'nt want to invest and then have to recoup the cost

A different scenario. The awesome Cruze has 150 bhp on tap, does a very good 204 kph. A different review comparing the Accord and the Passat TDi
did rate the Accord as more sprightly but the Passat with only 140 bhp from the VW TDi (CRDI) engine was also able to top at 240 (actual).The VW did have the DSG tranny as I do recall..Makes the Cruze look small dose'nt it. And that does make a difference to the buyer who thinks he's got the best and fastest or whatever. Now I know these are across segments (Passat TDI is in a different segment) but again, there is some amount of overlap because the executive versions of the Laura and the Superb also get similar spec engines.

What is the reason for this kind of a stark difference ? AFAI(Limited Knowledge)K , I can only think of less efficient vs. more efficient transmissions. But this is an important question to be asked , and where else to ask it other than team-bhp?I look forward to a good debate, please post your thoughts.. I am looking for the facts, as usual.

Note to mods: I wasn't sure whether to put it on Indian cars or in "Technical stuff" , so please move if you think it appropriate.
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Old 31st May 2010, 13:06   #2
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This is very similar to a thread I had created in "technical stuff".
Top speed depends on the final gear ratio a lot.
Hence a 140 bhp passat has a much longer final gear ratio than the 150 bhp cruze. This is designed in such a way, because the passat is made in such a way that it can handle those speeds, and the engineers are confident of giving it a ratio that can take the car to 240 kph.

What had intrigued me was the fact that the superb, inspite of being heavier, as well as 20 bhp down on power, was much faster to 60/100/150 than the accord. (this would be because it is more driveable (more torque at lower rpms), as well as having more gear ratios and faster shifts.)

Last edited by holysmoke : 31st May 2010 at 13:10.
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Old 31st May 2010, 13:10   #3
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I personally don't believe that 240kph bull of the passat. When I td'ed it, the car had a hard time getting to 160 let alone 240.
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Old 31st May 2010, 13:34   #4
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240km/h for Passat with just 140bhp sounds absurd. You are mistaken somewhere.
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Old 31st May 2010, 13:35   #5
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As per ACI test drive report, the Skoda Laura 1.8Tsi can do 205kmph in 6'th and 225kmph in 5'th. And my experience is similar - my Laura does do the 6'th gear numbers easily, whereas in 5'th it's much more aggresive surely telling me that it could comfortably go beyond the 6'th gear limit.

So if 6'th is taller than 5'th in the Laura 1.8Tsi, 5'th actually achieves higher top speed than the 6'th.

But then this is an insane engine - who knows I may reach the same top speed in 4'th as well if I hold...

Food for thought...

Last edited by anandpadhye : 31st May 2010 at 13:38.
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Old 31st May 2010, 13:53   #6
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Quote:
Originally Posted by anandpadhye View Post
As per ACI test drive report, the Skoda Laura 1.8Tsi can do 205kmph in 6'th and 225kmph in 5'th. And my experience is similar - my Laura does do the 6'th gear numbers easily, whereas in 5'th it's much more aggresive surely telling me that it could comfortably go beyond the 6'th gear limit.

So if 6'th is taller than 5'th in the Laura 1.8Tsi, 5'th actually achieves higher top speed than the 6'th.

But then this is an insane engine - who knows I may reach the same top speed in 4'th as well if I hold...

Food for thought...
yeah !!! hold it there , that engine will go for 210 + in 5th . my RS does thats , 217( odo reading) in 5th .
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Old 31st May 2010, 13:56   #7
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What manufacturers show you is the engine horsepower/torque. What actually matters is the wheel horsepower/torque.

So its possible that a car with lesser engine power could actually be producing more power at the wheels.

And i dont think gearing affects top speed to such an extent (unless you're running short of gears).

The car will achieve its top speed in the gear that has the ideal overall ratio (for that particular engine) and not necessarily the one with the tallest overall ratio. Which is why most cars achieve it in the one below top gear.

And what about aerodynamics? That would make the biggest difference in top speed.

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Old 31st May 2010, 14:49   #8
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This is a very interesting topic to discuss, and here are my thoughts on it.
(Am a newbie here, so please excuse in case of any errors and/or omissions on rules/protocols).

In a nutshell, BHP determines Top Speed, however, its not the only factor. The other factors (the list is not exhaustive):
  • Final- Drive ratio
  • Gear-ratios (Top 2 gears, specifically)
  • Aerodynamics - Higher the speed, more is aerodynamics' relevance. It is also a tight-rope between cutting drag and retaining/increasing grip.
  • Wheel size -Larger radius tyres would normally mean higher speed, at the cost of acceleration and vice-versa
  • Overall engine design (Breathing, cooling, revv-ability etc) - though arguably, this is more to do with the BHP the engine produces
  • Road-Surface quality and ambient weather conditions
  • Overall Chassis dynamics and suspension setup - The suspension setup, dampers' setup go a long way in determining car's high-speed road behavior, in turn influencing top-speed
  • Tyres' specifications - Rubber compound, construction, aspect ratio all can make a significant difference in top speed
Cheers,
AJ
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